1. Field of the Invention
The present invention relates to a doped multi-layer structure composed of a number of doped semiconductor thin layers stacked one on another. More particularly, the invention concerns a semiconductor device having a doping multilayer structure, i.e. multilayer structure formed by different doping.
2. Description of the Prior Art
A high electric conductivity of a semiconductor device implies that the device is excellent in high-speed operation and low power consumption. The electric conductivity is in proportion to the mobility of carriers and more specifically to the effective mobility of carriers in a current path. The effective mobility is usually lower than the bulk mobility of semiconductor which constitutes the current path, due to various scattering mechanisms. The bulk mobility, however, provides a reasonable measure for the effective mobility. Accordingly, it has been considered important for attaining a high-speed operation of a semiconductor device to realize it with a semiconductor material having a high bulk mobility. An example is a GaAs device in comparison to the Si device.
Further, there has been reported an attempt for modulating an electric signal or optical signal by applying an external electric field to a semiconductor multilayer structure including a doping superlattice, in the direction perpendicular to the structure and thereby varying the effective forbidden band gap of the doping superlattice. Reference may be made, for example, to K. Ploog and G. H. Dohler's article "Compositional and Doping Superlattices in III-V Semiconductors" appeared in "Advanced Physics" Vol. 32, No. 3 (1983), p.p. 285-359. In this literature, an nipi-doping multilayer structure in which doping concentration of both n- and p-type impurities is approximately on the order of 1.times.10.sup.18 cm.sup.-3, n.apprxeq.p.apprxeq.1.times.10.sup.18 /cm.sup.3, is discussed. In this structure, electrons are present in n-type layers, while holes are present in p-type layers. As a whole, both electrons and holes exist concurrently within the multilayer structure. This sort of structure is unlikely to promise the possibility of increasing the mobility.